This invention relates to an improvement in techniques for the manufacture of hollow, fiber-reinforced articles, and more particularly it pertains to an inflatable mandrel to be used in forming the interior surface of molded hollow articles.
This invention is an improvement in the type of molding apparatus described in U.S. Pat. No. 2,977,269 to C. M. Nerwick and U.S. Pat. No. Re. 25,241 to D. W. Randolph. According to the methods described in these patents, hollow cylindrical, fiber-reinforced plastic tanks have been successfully and economically pressure-molded. Generally, these methods include the steps of laying up fiber matting in a rigid mold casing in the approximate form of the desired article. A complete fiber form may include preformed matted fiber end wall caps which telescope into a laid-up, cylindrical sidewall portion.
An expandable bag or envelope which will define the interior shape of the finished article is positioned within the laid-up form in the mold. Rigid casing caps which shape the end walls of the final article are clamped to the ends of the mold casing to enclose the fiber mat form. With the fiber matting and mold thus assembled, the fiber matting is placed under a suitable moderate pressure by expanding the bag to hold it in place against the mold. The fiber matting is then permeated with a thermosetting resin or the like. The bag, besides defining the interior surface of the molded tank and determining the proper fiber-to-resin ratio, compresses the fiber lay-up in such a manner as to avoid migration of fibers with resin flow and resulting destruction of the lay-up.
The use of an expandable bag to compress the fiber lay-up and to form the interior shape of the molded article has been very successful in the production of fiber-reinforced hollow articles. However, more rigid and precise industry standards have necessitated even more accurate control of wall thickness and fiber-to-resin ratio.
According to such prior art techniques, a rubber bag is inflated to a predetermined pressure or series of pressures. The final volume occupied by the walls of the molded article depends principally on the bulk factor of the fiber lay-up, its compressibility, and the ultimate bag inflation pressure. To obtain a predetermined fiber-to-resin ratio in the finished molded article, it is necessary to control the compressed thickness of the fiber lay-up during resin injection and during subsequent molding operations. Variations from such an ideal or optimum final wall section thickness are the result of unavoidable slight differences in fiber lay-up assemblies and variances in inflation pressure. To some extent, the expandable bag does not correct but, rather, complies with deviations of the fiber matting from a desired final wall configuration.
When molding cylindrical hollow articles with domed, integral end walls, it is particularly difficult to maintain uniformity in the thickness of the fiber lay-up over the sidewall and end walls of the article. A given inflation pressure produces interdependent forces on the sidewall and end walls. Thus, it is difficult to adjust the inflation pressure to correct the force distribution on either the sidewall or end walls without affecting the other. If the expandable bag is inflated to a pressure where the end walls are sufficiently compressed, the sidewall of the article may be over-compressed. Over-compression of the sidewall may result in an unacceptably thin sidewall and/or a resin-starved sidewall and resin-rich end walls due to over-compaction of the sidewall lay-up and a resulting high concentration of fibers in the sidewall.